This summer I’ve tried my hand at gardening. Not many plants, but enough to brighten my day and provide some light snacks (strawberries) and garnishes (basil) for meals. I wouldn’t call myself a farmer by any means, but maybe someday I’ll work up to that status. There are some insects, on the other hand, that truly deserve the title of farmer. These are usually social insects such as ants, termites, or ambrosia beetles. For example, leaf cutter ants aren’t using those bits of leaves to feed themselves directly. They are feeding those plant bits to a fungus that they are cultivating within their nest and eventually they’ll feed this fungus to their larvae. This cultivating of microbes for food production is known as cultivation mutualism and there are few examples of fungal farming by non-social insects.
A recent research article in PlosOne discusses one such remarkable insect and it just happens to be a beetle! The species is Doubledaya bucculenta from the lizard beetle tribe Languriini (Coleoptera: Erotylidae: Languriinae). Just look at those fabulous forelegs! This species is endemic to Japan and uses those enlarged forelegs to hang onto dead bamboo shoots where females will make a small hole in the spring, lay an egg into this hole, and then plug this excavation with bamboo fibers. An adult beetle will emerge from this bamboo the following spring. Researchers noticed that the inner surface of the bamboo from which these beetles emerged was always covered in a white substance. So they investigated whether this might be an example of cultivated mutualism in a non-social beetle. They isolated the white fungus-like powder and extracted DNA from it to discover that it was the yeast species, Wickerhamomyces anomalus. The researchers also performed rearing experiments in plates with and without this specific yeast added. Interestingly, larvae that were reared with the yeast added grew to adults of normal size while those without the yeast died before reaching adulthood.
So, pretty convincing evidence that these beetles need this fungus to survive, but how are they actually getting this fungus? The researchers looked at bamboo shoots without larvae within and there were no traces of the white powder, so the fungus isn’t already within the bamboo. Somehow the beetles are getting this fungus in there when the females lay their eggs. A little further research on the morphology of the adults revealed that the females have a pouch at the end of their abdomen (males lack this pouch). Right around the area where females have their ovipositor (egg laying device). So perhaps some yeast cells are placed into the bamboo excavation when the females are laying their eggs.
The researchers found some yeast-like particles within the pocket (see figure D above), from which they again sequenced the DNA. The DNA sequences came back identical to the same species of yeast, Wickerhamomyces anomalus. So neat!
I found this article interesting for several reasons, most importantly because the researchers answered so many aspects of the specific question that they were after. As I was reading the article, I was thinking “but wait, do the beetles need the fungus to survive?” and “but wait, how does the fungus actually get there?”. And it was delightful that the researchers actually answered these questions in one fell swoop with this paper. The article contains lots of great general information on cultivated mutualisms in other insects, so I definitely recommend reading it. I’m sure there are thousands and thousands of beetle species out there with equally interesting natural history stories to tell. We just need people to research them!
Toki W, Tanahashi M, Togashi K, Fukatsu T (2012) Fungal Farming in a Non-Social Beetle. PLoS ONE 7(7): e41893. doi: 10.1371/journal.pone.0041893